Gardening in Colorado: zones, frost dates and when to plant

Colorado’s growing regions

The Colorado Climate Center at Colorado State University describes a state whose local climate depends mostly on elevation. The terrain falls into a few broad types, and each needs a different approach to vegetables.

• Eastern Plains. The high plains take up the eastern part of the state and rise gently westward from the Kansas and Nebraska lines. Expect low humidity, lots of sun, frequent wind and big swings between day and night temperatures. Most of the year’s rain falls during the growing season, often in thunderstorms.
• Front Range urban corridor. The plains meet the foothills here, and this is where most Colorado gardeners live. Chinook winds coming down off the mountains can raise winter temperatures sharply in a short time. CSU Extension’s planting advice often uses the Front Range as its reference area.
• Mountains and high parks. Above about 7,500 feet, CSU Extension treats a garden as a mountain garden. Temperature falls roughly 3.5°F with every 1,000 feet of elevation, so the frost-free season shrinks quickly as you climb. Cold air also sinks, so a valley floor can be colder than the hillside above it.
• Western valleys. The country from Delta and Paonia to Grand Junction and Palisade is especially mild, and the climate center notes that apricots, peaches, wine grapes and sweet corn are all grown there. Precipitation in these valleys is low.

What limits Colorado gardens

Hail. The climate center counts parts of eastern Colorado among the most hail-prone places in the country, and summer thunderstorms often damage crops in the northeast. In the mountains, snow and soft hail can fall even in July and August. Floating row cover gives young plants some protection from light hail.

Dry air and wind. Low humidity dries out soil and seedbeds quickly. CSU Extension’s mountain fact sheet suggests watering when the top 2 to 4 inches of soil feel dry instead of on a fixed schedule. It also warns that seedbeds may need water twice a day in summer, and that row covers and windbreaks help.

Alkaline, low-organic soil. CSU Extension says many Colorado soils run from pH 7.0 to 8.3 because of free lime, calcium carbonate left behind by weathering in a dry climate. In these soils sulfur does little to lower the pH, and iron chlorosis is a major problem. The recommended fix is to build organic matter with compost and mulch, aiming for the 4 to 5 percent that CSU considers ideal. Many mountain soils start below 1 percent.

Salty amendments. Because of the state’s large livestock industry, manure-based composts are the most common kind sold in Colorado, and CSU Extension warns they are often high in salts. Plant-based compost is usually lower in salt. Either way, ask for a specification sheet before buying in bulk.

What CSU Extension recommends

CSU Extension’s Vegetable Planting Guide (CMG GardenNotes #720) times spring planting by soil temperature, not the calendar. Read a soil thermometer at 8 a.m. at a depth of 4 inches (6 inches for beans), and plant once the soil reaches each crop’s minimum. Many cool-season crops can go in once soil reaches about 40°F. Snap beans need about 55°F, and melons, cucumbers and squash need about 60°F.

The guide’s other key points:

• Cool-season crops do best between 60°F and 80°F and lose quality in heat. Plant them in early spring, then plant again in midsummer for a fall crop.
• Broccoli, cabbage and cauliflower are usually set out as transplants in spring. In the warmer parts of the state, the best quality comes from seeding them directly in midsummer, around early July on the Front Range, for a fall harvest.
• Melons, cucumbers and squash have roots that don’t tolerate disturbance, so sow them directly. Black plastic warms the soil and speeds germination.
• Tomatoes, peppers and eggplant are normally grown from transplants. In warmer areas they can also be direct-seeded.

Gardening in the mountains

CSU Extension’s Vegetable Gardening in the Mountains makes cool-season crops the core of a high-elevation garden: lettuce, spinach, kale, chard, carrots, beets, radishes, peas, potatoes and broccoli. Of the warm-season crops, only bush beans and summer squash succeed with any regularity. The fact sheet’s other recommendations:

• Choose varieties with the fewest days to harvest. Packet numbers usually come from warmer climates, so expect crops to take longer.
• Plant on a south-facing slope or in raised beds, which warm faster in spring.
• Lay hardware cloth under beds to stop voles and pocket gophers.
• Keep floating row cover over crops through most of the season.
• If your well was drilled after May 2, 1972, check its permit before planning to irrigate. Some rural properties have no outdoor water rights.

Before you plant

Use the frost dates for your city above as a starting point, then check soil temperature before you sow. Get a soil test before amending: CSU Extension recommends one, especially in the mountains, and CSU’s Soil, Water and Plant Testing Laboratory in Denver tests soil samples. Its results tell you whether your soil has free lime and how much organic matter it holds.